


Stuck with you

by Sin_Cognito



Category: Undertale (Video Game)
Genre: Bara Underfell Sans (Undertale), Knotting, M/M, Omorashi, Sanscest - Freeform, Underfell Papyrus (Undertale), Underfell Sans (Undertale), Watersports, heat - Freeform, kustard - Freeform
Language: English
Status: Completed
Published: 2021-03-02
Updated: 2021-03-02
Packaged: 2021-03-15 10:21:58
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,691
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29806974
Author URL: https://archiveofourown.org/users/Sin_Cognito/pseuds/Sin_Cognito
Summary: Sans and his BIG teddy bear boyfriend Red have been dating for a while, but Sans has never seen Red in heat as he's been told it could be dangerous for him because of his low HP. Hopefully nothing goes wrong.Don't worry, nothing goes wrong.
Relationships: Sans/Sans (Undertale)
Comments: 2
Kudos: 78





	Stuck with you

**Author's Note:**

> And finally the prize for the second winner of my 500 followers raffle on twitter!!! They asked for kustard with knotting and omorashi, so I combined that with Bara Red and heat to spice it all up!! I hope you like it!!! ♥♥♥  
> This isn't intended as dub-con, but you could read it as that since both Sans and Red are out of it because of the heat. Just a heads up for those of you who are very sensitive to it. Take care! ♥

"BROTHER, RED, I AM NOW GOING TO TRAIN WITH UNDYNE," Papyrus announced from the doorway to Sans' room. "I WILL RETURN IN THE EVENING!"

"okay paps, have a nice day," Sans said from where he was comfortably snuggled next to his boyfriend on his mattress.

"have a nice day, papyrus," Red added as the tall skeleton closed the door and left the lovers alone, only to come back less than a minute later.

"I FORGOT TO MENTION, THERE ARE LEFTOVER SPAGHETTI IN THE FRIDGE FOR YOUR LUNCH! BE NICE TO EACH OTHER WHILE I'M AWAY!" He concluded with a smooch on the top of Sans' skull.

"nice, thanks bro," Sans beamed as he waved his favorite brother goodbye as Red chuckled.

"you two are so cute together."

Sans nudged his lover playfully, then hit play on his computer and the movie resumed where he had paused it. Red used the opportunity to readjust his position and pulled Sans on his lap, spooning him as he wrapped his arms around Sans' smaller waist and let his head fall on top of Sans' skull. What a teddy bear... Though to be honest, Sans loved it when his big boyfriend held him like that, it made him feel so small and safe in Red's strong arms.

Red was huge. That was one of the first thing Sans noticed about him when they first met, that day the Underfell brothers showed up in the basement to Sans and Papyrus' house. To be fair, it was hard to not notice just how much bigger the foreign monsters were, Red had a good head over Papyrus, who was himself pretty tall already, especially compared to Sans. And let's not even talk about Red's brother, nicknamed Edge. It was a miracle he even fit under Sans' roof.

After the initial shock of having bigger, rougher looking versions of themselves show up unannounced in their basement, Sans and Papyrus welcomed the two newcomers in their house and that was when the monsters explained they were from a parallel universe they called 'Underfell', where everything was like in the Underground, but... darker. Meaner. Which was the reason why they had decided to run away from it. Of course, the solution was only temporary as their mere presence in this universe would unbalance the very fabric of existence, and soon the machine took them back to their original universe, leaving Sans and Papyrus to wonder if there was anything they could do to help them leave the place for good and live here in the Underground, where it was safer and they wouldn't have any need for so much Lv.

To this day, Sans didn't know the full extent of what the Underfell brothers had had to go through in their universe, but he shuddered just thinking about it.

What he knew however, is that he deeply cared about Red, and it didn't take long after Sans managed to tweak the machine so the brothers could live here permanently for him to fall for Red. Thankfully, it turned out that Red reciprocated his feelings, and they started dating.

All of that happened years ago now, and today their relationship was good and strong. After living in such a pacifist world for so long, Red had softened a lot, mostly thanks to Sans' love and his patient nature. He still had the occasional nightmare about his past life, but overall he slept better, he and his brother didn't yell at each other all day anymore, he felt safe enough to bring up an issue when something made him uncomfortable instead of keeping everything bottled up inside like before. He swore less, which Papyrus greatly appreciated since him, Red and Sans all lived under the same roof now.

Well, not officially, Red supposedly still lived with Edge in the capitol, something the tallest brother had really insisted on. But mostly, Red would come visit Sans in Snowdin in the morning and spent the day with him before shortcutting back to the capitol for the night. Edge would say it was because he wanted to make sure Red was safe and after living in Underfell, it was understandable to want to check on your only family. But deep down, Sans knew it was because otherwise Edge would feel lonely. Stars only knew then why the tall skeleton was so adamant in staying in the capitol. Red seemed happy with the situation anyway so it didn't matter to Sans, as long as his boyfriend was happy, he was good. And he did get to spend all day with his lover so he wasn't about to complain about the arrangement.

What became clear as time went on and Red softened, was how teddybear-like he actually was, for a lack of a better word. Sans just couldn't describe him any other way. At first, it was casual, platonic touches like a slap on the back or a pat on the top of Sans' skull, then when they started dating, it evolved into more affectionate gestures like hand holding and an arm wrapped around Sans' waist, and now that their relationship was so serious, cuddles, a _skele_ ton of cuddles. Anytime, anywhere, whenever, wherever. If the two skeletons were in the same room for more than 4.5 seconds, Red would automatically find a way to wrap his bigger body around Sans, some way or the other.

It surprised Sans at first, and to be honest, it used to make him slightly uncomfortable. Physical affection had never been his forte, and for a long time, Papyrus was the only monster Sans let himself be carried around by. Maybe because he loved and trusted Papyrus enough to know that he was safe in his arms. Overtime, the same happened with Red, Sans got more and more at ease with him and his hugs, his infatuation turned into love and now he had a hard time going even one full day without his boyfriend's touches.

Which included... intimate touches. It had taken them time to first do the do, but once they had, they quickly got addicted to sex. It was good, it felt great and Sans loved every second of having Red inside him, hot and warm and oh so perfect. They couldn't let themselves go as often as they would've wanted since Sans tended to be _loud_ and with Papyrus around it could get a little awkward, but on the occasions where Paps wasn't home...

Like right now...

"what's going on in that handsome skull of yours?" Red asked with a knowing grin, and Sans had to wonder how he managed to read his mind like that.

"welp, i just thought that since paps was gone, we could... you know?" He winked at Red, turning around on his lap and craned his neck to place a sweet kiss on the other's teeth.

"heh heh, yeah, i think i got whachu mean," Red chuckled, his low baritone voice sending shivers down Sans' spine. "c'mere."

The big skeleton wrapped his arms around his smaller lover and Sans pressed their mouths together for a passionate kiss that left him breathless in mere seconds. Red's tongue felt so right against his, and he tasted so good, Sans couldn't help the moans that escaped him. He wasn't usually _that_ needy, but he could already feel his magic pool down there and couldn't wait for the moment Red would lower his shorts and-

_Grrrllbllrbl_

The two skeletons stopped what they were doing at the ominous sound and stared at each other for a second before bursting into laughter. It must've been closer to lunch time than Sans had realized, for both their stomachs to rumble like that at the same time. They wheezed and giggled for a moment, unable to calm down as each time they would look at each other, another fit of laughter would leave them panting.

Finally, Sans managed to calm down enough to jump off the bed and shakily say he was going to get the spaghetti so they could eat and then resume their activity.

"can you find another movie to watch while we eat?" He asked from the doorway.

"sure. is porn ok? i'm horny now."

"yup!" Sans gave Red a thumbs up before closing the door behind him and going down the stairs to the kitchen.

He opened the fridge and got two plates of spaghetti that he placed in the microwave, but as he was about to press the button to reheat the pasta, he thought he heard a faint beep sound coming from somewhere around the living room. He stayed silent and sure enough, there was another beep. Leaving the spaghetti in the microwave, he left the kitchen and went to the living room, straining his ear canals for the ringing sound that reminded him more and more of the ringtone of his phone that he had lost at some point.

The noise sounded like it was coming from under the couch, but Sans crouched down and couldn't see anything but dirt and filth under there. As he made a confused face about where the damn sound could be coming from, he had the idea to literally plunge his hand under the cushions of the couch, feeling around blindly until he felt something soft and hard under the tip of his fingers. As the ringing came back, the thing vibrated and Sans pulled it out from under the cushions.

Oh wow, look at that, it was indeed his phone. All things considered, it didn't look too damaged or dirty, but more importantly it was actually ringing. Sans couldn't make out the caller's ID through the dirty screen, but he managed to pick up.

"hello?"

"SANS!" Edge's voice boomed next to his skull, making him flinch and fall back on his butt. "SANS, ARE YOU OKAY? I'VE BEEN TRYING TO CALL YOU FOR AN HOUR!!"

"yeah? i'm... fine?" Sans replied, confused. It was rare already that Edge would need to call him, but he actually sounded... worried. Why would he worry about him? "is everything okay?"

"OH THANK STARS, YOU'RE FINE..." Edge sighed, more for himself than for Sans, but then he cleared his throat and talked in such a deep and serious tone that even Sans didn't want to joke around. "LISTEN TO ME. RED FORGOT TO TAKE HIS HEAT SUPPRESSANT MEDS. I FOUND THEM STILL ON HIS NIGHTSTAND, UNTOUCHED. HE'S DUE TODAY. IS HE WITH YOU?"

It took a moment for Sans to process the information, but when he did, he felt a cold shiver run down his back. This was bad.

He had never been with Red during one of his heats, not because he didn't want to, but because Red wouldn't let him. Apparently, while heat just made monsters like Sans hornier than usual, it drove the Underfell monsters to a feral-like state during which they could get _rough_ as they would only focus on breeding with another monster, regarless of who they were or what their relationship was like. It usually involved a lot of pain, Red had told Sans when he was explaining why they should in no circumstances be together when Red was in heat. With Sans' fragile HP, it could be very dangerous.

Thankfully, with Alphys' help, they were able to produce a very effective medication that helped defuse heat's symptoms, not too long after Red and Edge came to their world and started to live with them. Before that, the Underfell brothers had just 'dealt' with their heats in their own way, whatever that meant, protecting each other during a vulnerable time and making sure neither hurt any loved ones. Still, they both had gratefully accepted the medicine as it helped them not become mindless breeding machines once every other month.

So to hear that Red had forgotten to take the medicine this morning... As much as Sans loved the guy, he knew how dangerous the situation was.

Edge called out Sans' name again, drawing his attention back to the conversation, and Sans realized Edge was still waiting for an answer.

"uhm, he's upstairs, in my room. i was in the kitchen downstairs," he managed to mutter. "he... he seemed fine this morning, maybe he-"

"SANS. DON'T. YOU KNOW HOW QUICK OUR HEATS CAN TAKE CONTROL OVER US."

"but-"

"LISTEN TO ME. YOU'RE GOING TO TELL RED _THROUGH THE DOOR_ THAT HE FORGOT HIS MEDS THIS MORNING AND HE'S DUE. IF HE'S NOT TOO OUT OF IT YET, HE WILL UNDERSTAND AND SHOULDN'T POSE A PROBLEM. HOWEVER, IF HE TELLS YOU THAT HE'S OKAY AND HE ATTEMPTS TO BARGAIN FOR YOU TO GET IN OR WHATEVER, YOU _DO NOT_ OPEN THAT DOOR. YOU KEEP SAFE ON THE OTHER SIDE OF IT. YOU JUST TELL HIM THAT I'M COMING, OKAY?"

Sans deflated. He wanted nothing more than to help his boyfriend, but he was powerless here, there was nothing he could do but listen to Edge and do as he was told.

"okay..." Sans mumbled, disappointed in himself. He should've kept track of Red's heat cycles, he should've reminded him he was due today, he should've checked if Red had taken his meds.

"YOU STAY SAFE, I'M ON MY WAY. JUST... DON'T OPEN THE DOOR."

Sans was about to reply when he heard a door slam hard against the wall upstairs, and he whipped his head up in time to see Red slowly come out of his room.

"SANS? WHAT WAS THAT? SANS?!" Came Edge's voice through the dirty speaker, but Sans couldn't reply, he couldn't say anything.

Red was in heat.

It was unmistakable. His carmine eyelights pulsed dangerously in his dark eyesockets and were trained on Sans, almost burning his SOUL with their intensity. He appeared to be slighty hunched over like an angry cat that was about to strike down a prey and Sans didn't dare move a finger.

Sans took a deep breath and instanly realized how that was a big mistake, as a tentalizing smell invaded his body and his head, making him feel dizzy and faint.

"SANS! SANS!!" Edge shouted again, his voice hoarse from yelling. "SANS, YOU NEED TO LEAVE THE HOUSE!!! NOW!!!"

"but he smells so good..." Sans could only whine as Red walked down the stairs so slowly it should be illegal.

A distant part in Sans' mind got alarmed at how quickly his magic pooled at his pelvis and formed a pussy that got wetter with every second Red looked at him like that, but he pushed that voice away. He wanted this, he wanted Red, he craved for his touch and his magic.

He dropped the yelling phone as Red finally stopped in front of him, crouching down until he was face to face with Sans.

" **you smell very nice** ," Red purred and growled at the same time, and just the tone of his voice was enough to make Sans let out a high-pitched moan. " **i wanna taste you**."

Sans couldn't react and found himself frozen in place under Red's burning gaze. Next to his feet, his phone kept yelling, but he couldn't bring himself to pick it up and reply. He felt almost disappointed when Red tore his eyelights away and noticed the phone on the ground, grabbing it in his large skeletal hand. Sans briefly wondered if he was going to talk with Edge even though he technically knew that in this state, Red wouldn't be able to hold an actual conversation, but Red just tightened his hold around the cellphone and crushed it into a million pieces.

Oh. Welp.

At least the yelling stopped.

Sans let himself be picked up by the bigger skeleton, melting in his arms as he unconsciously wrapped his legs around Red's waist for support. He moaned again when he felt Red's erection poking through his shorts right where his pussy made contact with the other's body.

Red stayed in the middle of the living room like that for a few moments, nuzzling Sans' crook of the neck as he carried him in his arms like he weighed nothing. The more his teeth gently grazed over the bones of Sans' neck, the more the smaller skeleton wanted to purr and moan and whine, but most of all, he wanted to beg for more. Even though they hadn't done anything yet, he still was eager for more... Sans distantly thought he should've been scared of the situation he was in, but his mind was too hazy to focus on anything else than Red's tentalizing smell.

Sans let himself be carried to his room and hummed as Red placed him surprisingly gently on his bed, before the bigger skeleton hovered over him. Oh, how Sans felt tiny and minuscule under him like that.

" **mine** ," Red growled as he began to sigh and pant over Sans.

No, wait, he wasn't just breathing rapidly, he was actually smelling Sans, sniffing him like a dog. Red kept smelling Sans' body, grunting more ' **mine, mine, mine** 's as he inhaled Sans' smell, growling lowly when he came near Sans' crotch. Sans knew he could smell the scent of his excited magic there, and without realizing it, he opened his legs further, hoping Red would get the message and finally take him. But Red propped himself back up on his arms and went back to sniff at Sans' skull.

" **you smell like _him_**. you're mine, mine!" Red snarled as he finally lowered his shorts to reveal his ruby cock that shone brightly in the dim room. It seemed bigger than usual, but at this point Sans couldn't say if it was his thirsty imagination or reality, so he just whined again. " **i won't let _him_ leave his scent on you like that**."

A warm stream of yellow-ish liquid hit Sans on the side of his skull, and soon he found himself covered with the warm liquid and could smell Red's scent all over him.

"red..." He couldn't help but moan as his pussy clenched just from inhaling his lover's scent. Damn it, Red hadn't even touched him yet! "red, please... take me... make me yours..." He managed to breathe out as the stream finally stopped and Red bent down again.

The massive skeleton started to lick and lap at Sans' wet bones, like he was grooming him and Sans purred at the treatment, too content to finally have his lover's tongue on him, not even noticing when Red used the opportunity to remove Sans' shorts. Sans wrapped his arms around Red's neck and managed to pull him back in for a kiss, and when Red's tongue entered his mouth, he wailed at just how utterly good it felt. He melted in the kiss as Red growled into his mouth, their tongues dancing against each other hotly, Red's broader one taking control of Sans' cyan appendage. It was a messy kiss, but Sans didn't mind the rougher treatment, not even when Red broke the kiss and Sans' tongue lolled out of his mouth, saliva dripping down his chin onto his shirt.

Too content to feel his lover's magic still on his tongue, Sans didn't pay attention as Red lined himself up with his entrance.

" **you're _mine_** ," was the only warning Sans got before Red slammed his cock all the way in in one go.

Stars was Red's cock enormous. If he had been in his right mind, Sans would've worried that Red might break him in half, but with how wet and turned on he was, Red's dick slid right in with no discomfort whatsoever. Quite the opposite. In fact, it took Sans less than half a second to be moaning like a bitch at just how good Red's cock pressing deep into his pussy felt.

"oh fuck, oh fuck, oh fuck!" He chanted as Red sped up the pace, making him see stars.

Sans could only lie there and take it all, his magic running wildly throughout his body while his pussy sent waves after waves of pleasure right to his SOUL. Red continued to thrust deep and hard, focusing on nothing else but chasing his own pleasure and Sans couldn't be more grateful for the rough treatment that pushed him over the edge barely one minute in.

His pussy clentched, his back arched and his body tensed as he let out a high pitched whine, but Red didn't stop. He hooked his arms under Sans' knees and lifted his legs until they were almost touching his ribcage. He grabbed Sans' flailing arms and pinned him on the bed before pounding into him even faster than before and just as deep.

" **mine, mine** ," Red kept growling as his cock reached the far end of Sans' magic, finding the sweet spot there that had Sans go wild and thrusting into it repeatingly.

"red oh my fuck!!!" Sans cried out as another orgasm hit him full force and made him tremble under Red's bigger form, tears forming at the corners of his eyesockets and threatening to spill out if Red kept slamming his cock into Sans like that.

Which he did, of course. Red's breathing was shallow and ragged and his thrusting more erratic though not any less deep or strong than before. Sans could feel the other was close, or at least he hoped. He wanted this, wanted to feel his lover's cum inside him. He wanted to feel so good he wouldn't be able to walk tomorrow. He wanted to be used like the bitch he was.

" **sans** ," Red managed to grunt before his whole body suddenly tensed and he let out a piercing howl.

Sans felt Red's release invade him, and watched with aroused fascination how ruby swirls formed under the blue of his summoned tummy, filling him to the brim with hot magic. Red gave a few more thrusts, pumping himself dry into Sans' pussy until he finally stopped, panting heavily above Sans.

Silence settled in for a moment, with Sans' and Red's soft breathing the only sounds breaking the quietness of the room, until there was a loud bang downstairs. Sans would've moved and gone downstairs to see what happened if he wasn't currently trapped under his big boyfriend, so utterly spent with his still summoned pussy filled to the brim with red and blue cum, and just plain too lazy to move after all the exercise. It didn't matter anyway as the source of the loud noise came to the room less than ten seconds later.

"SANS! RED!" Edge yelled with worry as he slammed open the door to Sans' room. "IS EVERYTHING OK- OH MY FUCKING STARS!!!" He shrieked when he saw the position Sans and Red were still in, turning around promptly but not leaving the room completely. "IS... ARE YOU... SANS, ARE YOU OKAY? PLEASE AT LEAST TELL ME YOU'RE OKAY. AND FOR THE KING'S SAKE, CAN YOU COVER YOURSELVES?!"

Sans couldn't help it and let out a chuckle, even with Edge's back turned to him he could see the violent blush on his sharp cheekbones, and the way his whole body was stiff. For being a skeleton nearly three times Sans' size, Edge could be so cute sometimes.

"i'm fine," Sans whispered, his voice more hoarse than anticipated.

He was about to reassure Edge further that Red hadn't hurt him at all when Red, who had been silently nuzzling Sans' neck up until now, spoke up.

"bro, can you leave us alone for a sec?" He said in a surprisingly neutral tone, any trace of the growling and grunting from earlier completely gone from his voice.

Was his heat already over? Was that all it took? Sans couldn't help but wonder why the Underfell brothers had made such a big deal out of their heats if some quick rough fucking was all it took to calm them down.

Edge let out a long sigh and didn't say anything for a moment, before he turned around again, though he never looked at Sans and Red and just averted his eyes the whole time. "OK. I'LL GO PREPARE WATER AND SOME FOOD FOR YOU TWO, AND THEN I'LL CALL PAPYRUS AND TELL HIM TO GO TO MY PLACE FOR THE NEXT FEW DAYS. I'LL BE DOWNSTAIRS. JUST... BE CAREFUL WITH SANS. HE'S FRAGILE."

"i promise," Red said, and then Edge left the room.

Ooookay what the fuck was that? Sans turned a confused look to his lover, who sported a serious expression on his face that was unusual enough to make Sans worry. Red wrapped his arms around Sans' waist and pulled him upright so he was straddling Red's lap, and only then did Sans realize that Red's cock was still buried inside his magic. That wasn't normal, both their ecto-bodies should've dissipated by now. He was about to ask what was going on when Red pulled Sans' soaked tee-shirt over his head and threw it carelessly in the general direction of the pile of dirty clothes in the corner of the room.

Now that he was less out of it, Sans could smell (and _feel_ ) he was still very much covered in pee despite Red's grooming from earlier. For some reason he hadn't minded it before but now he couldn't help but make a face that Red noticed.

"erm, sorry about that," the big skeleton bashfully said, looking ashamed of himself.

"it's okay, i'll just go wash myself," Sans just said and was about to stand up when Red grabbed him and pulled him down.

"sorry babe, but that ain't happening anytime soon," he said and gestured at their linked magic and indeed Sans could see how Red's cock was swollen at the base, effectively preventing him from pulling out of Sans without making at least one of them very, _very_ , uncomfortable.

"what even the fuck is this?" Sans couldn't help but ask.

"that's... my knot," Red replied after a long sigh. He then placed a gentle hand on Sans' cheek and stroked him lovingly before he continued in a serious tone. "listen to me sans. i'm still in heat and this is very much not over. not at all. from now on and until my heat passes, we're linked together. right now i'm satisfied, so my scent isn't as strong as earlier and we can both think clearly. but in fifteen minutes or so, i will get horny again and only then will my knot go away so i can pull out and go another round."

Well then, that just meant Sans had to wait fifteen minutes before Red would pull out and let Sans go, right? That didn't sound so terrible.

"so then-" Sans began but Red interrupted me.

"i know what you're thinking and no, you won't be able to leave then. because i'll be horny again and my scent will be strong again and we'll both be out of it again. and if you couldn't stop me from fucking you earlier, you sure as hell won't be able to now as my scent will only go stronger each time i climax. and i will, erm... get rougher too."

"oh," Sans could only say.

Welp, that explained why Red had never wanted Sans to be present during his heats. He never doubted that Red would never hurt him, but Red _in heat_? Now that he had first hand experience, he didn't know how far it could go, how much rougher the big skeleton could get in the heat of a... well, heat. Heh heh.

Then again, it wasn't like he had a choice, if he was either linked to Red because of his knot or too out of it to leave, what could he do about it? No matter how much he thought about it, it seems like they were just...

Just...

"sans, i'm so sorry it ended up like this," Red said quietly, guilt written all over his face. "i never wanted you to see me in heat because I-"

Sans couldn't help but burst out laughing, interrupting his lover's sentence.

"so," he managed to breathe out between two giggles, "what you're saying is that, heh heh heh, basically, we're _stuck_ together until your heat passes."


End file.
